1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of freely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
Despite the long existence of the FCC process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e. yield and selectivity. Two facets of the FCC process that have received attention are recovery of adsorbed products from the spent FCC catalyst and initial contacting of the FCC feed with the regenerated catalyst. Improvement in the recovery of hydrocarbons from spent catalyst and better initial feed and catalyst contacting improves the yield and selectivity to selectivity to more valuable products.
The processing of increasingly heavier feeds and the tendency of such feeds to elevate coke production and yield undesirable products has led to new methods of contacting FCC feeds with catalyst. Of particular interest recently have been methods of contacting FCC catalyst for very short contact periods. U.S. Pat. No. 4,985,136 discloses an ultrashort contact time fluidized catalytic cracking process, the contents of which are hereby incorporated by reference that contacts an FCC feed with a falling curtain of catalyst for a contact time of less than 1 second followed by a quick separation. U.S. Pat. No. 5,296,131 the contents of which are hereby incorporated by reference discloses a similar ultrashort contact time process that uses an alternate falling catalyst curtain and separation arrangement. The ultrashort contact time system improves selectivity to gasoline while decreasing coke and dry gas production by using high activity catalyst that contact the feed for a relatively short period of time. The inventions are specifically directed to zeolite catalysts having high activity. The short contact time arrangements permit the use of much higher zeolite content catalysts that increase the usual 25-30% zeolite contents of the FCC catalyst to amounts as high as 40-60% zeolite in the cracking catalyst. These references teach that shorter hydrocarbon and catalyst contact time is compensated for by higher catalyst activity.
In traditional long contact time FCC systems, it has been known to recycle catalyst from the end of a conversion zone that contains coke deposits, i.e., spent catalyst, back to the bottom of a reactor zone. Examples of long contact time risers that use this type of arrangement are shown in U.S. Pat. No. 3,679,576 where spent and regenerated catalyst pass together momentarily through a short section a relatively small diameter conduit before contacting the FCC feed. The contacting of spent catalyst, regenerated catalyst, and feed has been shown to occur simultaneously in U.S. Pat. No. 3,888,762 where all components come together simultaneously in a riser conduit. These types of arrangements have not been successfully practiced in commercial units.
Thus, in FCC operation generally and particularly in the short contact time operation, maximization of feedstock conversion is ordinarily thought to require essentially complete removal of coke from the catalyst. This essentially-complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 weight percent coke. In order to obtain complete regeneration, oxygen in excess of the stoichiometric amount necessary for the combustion of coke to carbon oxides is charged to the regenerator.
While the prior art has recognized that the potential benefits of short contact times in high activity catalyst in FCC processing arrangements, little attention has been paid to the catalysts circulation aspects of the process. Ultrashort contact times will reduce the amount of catalytic coke deposited on the catalyst. Operating an ultrashort contact time FCC process with complete regeneration will increase the total mass of solids circulated for the combustion of a given amount of coke. This effect will produce lower regenerator temperatures. Increasing the total amount of solid circulation through the reactor and regenerator for the combustion of a fixed amount of coke will adversely affect the kinetics within the regeneration zone. Circulating large amounts of catalyst with low coke concentrations unnecessarily increases the amount of mass circulated throughout the unit.